Austenitic stainless steels

ABSTRACT

AUSTENITIC STAINLESS STEELS CONTAINING 0.05-0.15% C, 0.3-1.0% SI, 13.0-16.0% CR, 4.0-12.0% MN, 0.5-3.0% NI, 0.05-0.20% N AND THE BALANCE OF FE AS THE ESSENTIAL INGREDIENTS.

United States Patent US. Cl. 75-125 1 Claim ABSTRACT OF THE DISCLOSURE Austenitic stainless steels containing ODS-0.15% C, 0.3-1.0% Si, 13.016.0% Cr, 4.0-12.0% Mn, 0.53.0% Ni, ODS-0.20% N and the balance of Fe as the essential ingredients.

The present invention relates to austenitic stainless steels containing small amount of Ni, particularly to those remarkably improved in formability and corrosion resistance, which are characterized by containing as the essential ingredients ODS-0.15% C, 0.31.0% Si, 13.0- 16.0% Cr, 4.0-12.0% Mn, 05-30% Ni and ODS-0.20% N and the assumption of the fully austenitic phase after annealing, more particularly to those characterized by containing 1.0-4.0% Cu in addition to the above mentioned ingredients.

As is known to those skilled in the art, 18% Cr, 8% Ni austenitic stainless steels have been developed by the addition of a large amount of Ni content in order to stabilize the austenitic phase at a room temperature. They have the advantages of improved mechanical property, formability and corrosion resistance, etc., and the disadvantage of the inevitable large amount of Ni content. Tenelon steel containing the 18% Cr, 15% Mn and 0.7% N is one example of austenitic stainless steels for the purpose of replacement of the Ni with Mn and N which can also produce austenitic structure. However, said steel may produce blow-holes at the time of ingot making and requires an unusual steel making process and an elevated atmospheric pressure at the time of melting process due to the disadvantages derived from the inevitable high N content, thus unsuitable for press forming etc., and it has not been used in practice yet.

Recently, AISI types 201 and 202 which are used as the substitutes for AISI types 301 and 304 have been developed to replace a part of the Ni with such elements as Mn and N which can produce austenitic structure. These AISI types 201 and 202 are stainless steels containing 35-55% and 4-6% of Ni respectively, therefore these types stainless steels contain relatively high Ni contents and are inferior to AISI 301 and 304 in formability. This is because the relative contents of C, Si, Mn, Cr, Ni and N of AISI type 201 composition are derived from the view point of the inhibition of the occurrence of a-ferrite structure. This type of austenitic phase is very stable against martensitic transformation. Accordingly, this type of steel loses one of superior characteristics of austenitic stainless steels, stretch formability which is based on the high work hardening property as a result of a partial transformation of austenitic phase into martensitic phase in the forming process. Further, AISI types 201 and 202 products are hard and preferable for use in a part of a structural product but undesirable for press-forming process due to the increased spring back and occurrence of Wrinkle often observed on some shapes of products. It has now been made possible, as a result of our study on Ni restrained stainless steels, to provide fully austenitic stainless steels by replacing a part of the Ni with such elements as Mn, N and C which can produce austenitic structure, thereby effecting the ice reduction of Cr content to less than 16% and the Ni content to 0.5-3.0%, thus breaking down a prior such recognition in the field of stainless steels that the reduction in Cr content brings about a considerable decrease in corrosion resistance. Thus found definite region is equivalent in its properties to conventional 18-8 stainless steels not only in corrosion resistance but also in workability in spite of the restraint in the Ni content, thereby eliminating some disadvantages due to the high Cr and Ni content.

That is to say, the stainless steels according to the present invention are the austenitic stainless steels consisting essentially of 0.05-0.15 C, 13.016.0% Cr, 0.31.0% Si, 4.012.0% Mn, ODS-0.2% N, (LS-3.0% Ni and the balance of Fe and other incidental ingredients, and if desired, further addition of 1.0-4.0% Cu. They are fully austenitic phase in the annealed condition and may produce some martensite or remain to be fully austenitic phase by deformation. They have extremely improved mechanical properties, forming properties and corrosion resistance.

C is a productive composition for austenitic structure as well as N, in the case of the stainless steels according to the present invention. It is desirable, however, to keep the C content less than 0.15% since the excessive amounts of C will cause the precipitation of the carbide at the time of cooling after Welding and annealing, thus reduces the intergranular corrosion resistance.

Cr content must be kept to less than 16%, because the excessive amounts cannot produce the fully austenitic phase that is the essential feature of the present inven- )tion. On the other hand, the corrosion resistance decreases sharply in the region less than 13%, therefore Cr content is to be not less than 13%.

So far as the corrosion resistance is concerned, higher Si content is preferable, but it has to be kept at less than 1%, since excessive Si content will cause the formation of 6-ferrite structure and the reduction in hot workability.

Mn content must be more than 4%, for the less content cannot retain the fully austenitic phase in the steels. It is desirable to keep the contents of the Mn ingredient within such a lower range that can retain austenitic structure, for the increased amount is less productive for austenitic phase, accelerates the oxidation at high temperature under hot working and annealing, and causes substantial reduction in good surface properties of finally obtained products.

Although the increased content of Mn have an advantage the superior stability of austenitic phase, the contents must be kept to less than 12%, for the excessive contents have some disadvantages over the above mentioned advantage, such as the reduction in return of the scraps and the accelerated oxidation at high temperature.

Ni is an essential ingredient in order to reduce the contents of N, C and Mn those can produce austenitic structure, and to retain the stable austenitic phase. The Ni contents must be kept to no more than 3% and not less than 0.5%, because the effect does not appear within the ranges less than 0.5% and in a range more than 3%, the effect does not increase proportionally to the increase in Ni content. Small content of Ni as in the present invention improves the corrosion resistance and the hot workability of Cu added to austenitic stainless steels wherein Cu is added to provide a preferable characteristic to make the steel suitable for use in press forming. When Ni content is not within the range of the present invention, Cu will come to the surface in the steels under hot rolling by selective oxidation of Cr, Mn or Fe ingredients and therefore, the fine cracks would occur on the surface of the steels at the time of hot rolling and the above mentioned fine cracks would remain during the cold rolling and large amount of grinding work will be required for the coil cracks grinding process in order to eliminate the above mentioned fine cracks. The stainless steels according to the present invention can prevent the occurrence of the fine cracks on the surface of the steels at the time of hot rolling and the undesirable influences due to the incorporation of Cu by the addition of Ni.

N is a productive composition for stabilizing austenitic structure. However, even if the content of N is less than solubility blow-holes will often occur by the inter-action between N and Hydrogen existed in the steels, and bleeding phenomenon of the ingot may be observed.

According to the conventional steel making processes, -8 p.p.m. of hydrogen content is inevitable in the steels Table 1 illustrates some examples of chemical compositions and the amounts of martensite after 40% tensile deformation for the steels of the present invention, the referenced steels and the conventional steels. As seen from these examples, the content of Ni and N in the steels of the present invention is lower, however, these steels can be easily transformed into the fully austenitic phase without any increase in the Cr content more than 16%. And they include the metastable and stable austenitic stainless steels; the former being those in which a part of the fully austenitic phase has been transformed into martensite after deformation and the latter being those in which all the fully austenitic phase remain untransformed even after deformation.

TABLE 1.EXAMPLES OF CHEMICAL COMPOSITIONS AND AMOUNTS OF MARTENSITE AFTER 40 PERCENT TENSILE DEFORMATION FOR THE STEELS REFERENCED STEELS AND CONVENTIONAL STEELS ACCORDING TO THE PRESENT INVENTION,

Chemical composition (percent) Amount of martensite Description Type number Grade designation 0 Si Mn Cr Ni N Cu (percent) Steels oi the present invention Y-54 14 Cr-lO Mn-Ni 0.10 0.53 10.19 14.26 1.58 0.14 1.4 Y-57 Or-6 Mn-Nl 0.11 0.44 5.72 14.82 2.84 0.09 37 Y-2 15 Cr-8 Mn-3 C11-N1 0.11 0.47 8.11 15.11 2.51 0.09 2 tr Y-B 15 Cr-8 Mn-l Cu-Nl 0.12 0.55 8.20 15. 23 1. 62 0.14 1. 3.5

NY-5 15 Cr-7 Mn-2 Cu-Ni 0.11 0.51 6.90 15.26 0.75 0.09 2. 17

NY-7 14 Cr-5 Mil-3 Cu-Nl 0.08 0.54 5.41 14.20 1.42 0. 12 2. 23

NY-8 14 (Jr-12 Mn-2 Cu-Nl 0.10 0.47 11.72 13.80 1.78 0.06 2. tr

Referenced steels- N-15 14 Cr-14 Mn-2 Cu 0.13 0.48 13.40 14.13 0.19 1. tr H-37 15 Cr-10 Mn-2 Cu 0.10 0.44 10.19 14.82 0.14 1. tr

Conventional steels AISI-430 17 Cr 0.07 0.47 0.28 16.60 0.03 AISI-301 17 Cr-7 Ni 0.11 0.57 0.99 17.20 0.01 46 AISI-4304 18 Cr-8 Ni 0.08 0.59 1.06 18.38 0.01 tr AISI-201 17 Or-6.5 Mil-4.5 Ni 0. 10 0.43 6.61 17.13 0.14 tr AISI-202 18 Cr-9 Mn5.5 Ni 0.07 0.51 9.13 17.92 0.14 tr obtained, thus in order to make a sound ingot at the condition of the above mentioned content of Hydrogen, N content must be kept in a range of less than 0.2%.

Cu has an effect to impart to the steels of the present invention corrosion resistance and to furnish a desirable characteristic to make the steel suitable for use in the press forming through its softening effect. And Cu has the action to decrease the contents of C, N, Mn and Ni, the said action being over Mn and almost equivalent to Ni. Cu content is to be kept within the range of 14%, since the excess Cu content causes the coppers red-shortness and adversely affects the hot workability.

Of course, the rare-earth elements, Ti, B or Nb which are conventionally used as the so-called additional metals Table 2 illustrates some mechanical properties, formabilities, and spring back characteristics of the steels according to the present invention, the referenced steels and the conventional steels in the prior art. From these values it is clearly understood that some of the steels according to the present invention are equivalent to AISI 301 in stretch formability. They belong to the metastable stainless steels in which a part of the fully austenitic phase has been transformed into martensite after deformation. And it is also clearly understood that some of the steels according to the present invention which include Cu have the coppers softening effect and the lowered yield strength indicated by yield stress and hardness. These steels are improved in the spring back characteristic and formability.

TABLE 2.MECHANICAL PROPERTIES AND FORMABILITY TEST RESULTS FOR THE STEELS ACCORDING TO THE PRESENT INVENTION, REFERENCED STEELS AND CONVENTIONAL STEELS. (SAMPLE THICKNESS 0.8 MM.T.)

Tensile test For-inability test I Elon- Conical Spring Hard- Yield Tensile gation cup Erichsen back Type ness, strength strength (pervalue value angle Description Number Grade designation Hr(10) (kg/111ml) (kg/111111. cent) (percent) (Percent) th r t invention Y-54 14 Cr-10 Mn-Ni 178 38 73 59 23. 8 13. 1 Steels of e p esen Y-57 15 Cr-G Mn-Ni 193 36 84 57 23. 2 13.8 3. 1 Y- 15 Cr-8 Mn-3 Cu-Ni 155 31 71 59 24. 5 14. 5 2. 7 Y-3 15 (Jr-8 Mn-l Cu-Ni 196 39 97 59 22. 9 12. 4 3.0 NY-5 15 Or-7 Mn-2 Cu-Ni 173 33 87 57 23. 5 13. 5 NY-7 14 Cr-5 Mn-3 Cu-Ni 175 33 87 59 23. 5 14. 0 2 8 NH a a a2 a as a: ed teels N-15 14 r-14 n-2 u Rem-em S H-37 15 Cr-10 Mn-2 Cu g? i lsteels AISI-430 17 Cr Convent om AISI-301 17 Cr-7 N1 160 28 81 61 24. 4 14.6 3. 1 AISI-304 18 Cr-8 Ni 160 30 66 58 24.0 12.1 3. 0 AISI-201 17 Cir-6.5 Mn-4.5 Ni 197 36 76 59 23. 5 11.9 AISI-202 18 Cr-9 Mn-5.5 Ni 176 35 70 58 23. 8 11.9

1 Spring back angle=Spring back angle after releasing the right angle best plate.

may be added as the incidental ingredients in the range of up to 0.1% for Ti, up to 0.1% for Nb, and up to 0.005% for B to improve hot workability or some other properties.

The steels according to the present invention will be explained more in detail by the following Tables 1 to 4, wherein the referenced steels in the tables are to illustrate differences between the stainless steels of the present invention and those free of Ni composition.

TABLE 3.CORROSION RESISTANCE TESTS F B. THE STEELS ACCORDING TO THE PRESENT INVENTION, REFERENCED STEELS AND CONVENTIONAL STEELS Salt water immersion test 1 24 hours 2 48 hours 1 Description Type number Grade designation RN. A.N. RN. A.N.

Steels oi the present invention Y-54 14 Cr-10 Mn-Ni 6 4.1 6 5. 4 Y-57 15 Cr-6Mn-Ni 6 4. 1 4. 1 Y-2 15 Cr-8Mn-3Cu-Nl 9. 9 10 9. 9 Y-3 Cr-8Mn-1C u-Ni 9 8. 1 9 8. 8 NY-5 15 Cr-7Mn-2Cu-Ni 10 9. 9 10 9. 9 NY-7 14 Cr-5Mn-3Cu-Ni 10 9. 9 10 9. 9 NY-8 14 Cr'l2Mn-2Cu-N i 10 9. 9 10 9. 9 Referenced steels N15 14 Cr-14-Mn-2Cu 9 8. 4 10 9. 2 111-37 15 Cr-10 Mn-2 Cu 10 9. 9 10 9. 6 Conventional steels 11181-4550 17 Cr 4 2. 1 5 2. 4 AISI-301 17 Cr-7 Ni 5 4. 1 6 5. 0 21181-1304 18 Cr-8 Ni 6 4. l 6 5. 4 AISI201 17 C1' 6.5 Mil-4.5 Ni AISI-202 18 Cr-9 Mid-5.5 Ni 10 9. 9 10 9. 0

A solution prepared by mixing a solution consisting of 0.5 g. of sodium sulfate, 0.25 g. of sodium sulfite, 0.1 g. of sodium thiosulfate, 52.5 g. of sodium chloride and 525 cc. of water with a solution consisting of 52.5 g. of calcium chloride and 525 cc. of water.

2 Test in an air containing 0.66% by volume of S02 concentration. Table 4 illustrates the mean depth of cracks in coil cracks grinding process on some of the Cu-included steels according to the present invention and the referenced steels including Cu and free of Ni. As seen from the table, even the steels including Cu remarkably reduce the depth of cracks by the addition of small amount of Ni.

TABLE 4.MEAN DEPTH OF SURFACE CRACKS OF THE STEELS ACCORDING TO THE PRESENT INVENTION AND 1;HE REFERENOED STEELS (SAMPLE THICKNESS Mean . depth of the Description Type number Grade designation cracks (p) Steels oi the present Y-Z 15 (Jr-8 Mn-3 Cu-Ni 5 invention.

Y-3 15 Cr-8 Mn-l Cn-Ni 6 NY-5 15 Cr-7 Mn-Z Cu-Ni 8 NY-7 14 Cr-5 Mn-3 Cu-Ni 6 N Y-3 14 (Jr-12 Mn-Z Cu-Ni 3 Conventional steels. N-15 14 Cr-14 Mn-2 Cu 60 H-37 15 Cr-lO Mn-Z Cu 63 3 P.N.=Protection number.

4 A.N.=Appearance number.

5 Number of pit observed on a sample of mm., 100 mm. Number of pit observed on a sample of mm., 110 mm.

1. Austenitic stainless steels consisting essentially of (A) 0.05 to less than 0.15% C,

(B) 0.3 to 1.0% Si,

(C) less than 16%, but not less than 13% Cr,

(D) 4.0 to 12.0% Mn,

(E) 0.5 to 3.0% Ni,

(F) 0.05 to 0.20% N,

(G) 1.0 to 4.0% Cu,

(H) at least one additional ingredient selected from the group consisting of up to 0.1% Ti, up to 0.1% Nb, and up to 0.005% B, and

(I) the balance Fe as the essential ingredient, said steels being characterized by the assumption of the fully austenitic phase after annealing.

References Cited UNITED STATES PATENTS 3,112,195 11/1963 Soureshy 128 A 3,152,934 10/1964 Lula 75128 A 2,657,130 10/1953 Jennings 75128 A 2,138,289 11/1938 Becket 75-128 A 2,687,955 8/1954 Bloom 75-128 A 2,747,989 5/1956 Kirkby 75-128 A 2,799,577 7/1957 Schempp 75-128 A 3,562,781 2/1971 Tanczyn 75128 A 50 HYLAND BIZOT, Primary Examiner U.S. Cl. X.R.

75128 A, 128 N, 128 T 

